1. Technical Field
This invention generally relates to methods and devices for applying solder to workpieces and more particularly to a method and an apparatus which uses a molded selective solder pallet to allow selective application of solder to a circuit board with a wave soldering device.
2. Background
Many electronic devices use a circuit board as the foundation building block for the electronic circuitry which forms the device. With the advent of the printed circuit board (PCB), the board itself actually became part of the circuit, forming conductors for the electrical current between the various electronic components. The circuit board both supports the electronic components and links them together. Generally, the circuit board consists of a nonconductive base, such as fiberglass, coated with a thin layer of a conductor, such as copper, which is etched to form a pattern of electrical conductors.
Soldering of the various electronic components to a circuit board is generally accomplished by one or more of the following processes: manual soldering, reflow soldering, discreet component soldering using a solder pot and flow well or continuous soldering using a wave soldering machine.
Wave soldering has long been the soldering method of choice for high production circuit boards which require through-hole parts. This is a continuous process in which circuit boards are passed over a wave of molten solder. The solder wets up through metal plated holes containing the component leads to consummate the soldering process. While this process lends itself to automation, as the parts are simply loaded onto a conveyor which in turn passes the parts over a wide wave of solder, it also imposes restrictions in terms of which parts can be exposed to the high thermal shock of a molten solder wave. For instance, it is oftentimes desirable to mix surface mount components and through-hole components on the same board. However, it usually isn't desirable to expose a surface mount IC components directly to a solder wave as thermal damage to the component may result. Additionally, the connections of surface mount components are so close together that solder can easily bridge two or independent more connections. Consequently, areas on the side of the board which cannot be exposed to a wave, and which otherwise would be wave soldered, are soldered by some other method.
Further advances in PCB technology have resulted in dual sided PCBs, to which components are attached on both sides of the boards, allowing intricate and complex circuits to be implemented in a smaller footprint. These doublesided and mixed technology assemblies may employ active components including integrated circuits (IC's) and memory devices packed tightly on both sides of the PCB. In such cases, industry practices specify that either a custom design flow well on the solder pot be used or a wave solder machine equipped with a special baffle to expose only those areas requiring solder. The latter has proven to be a slow process which only may be accomplished after in-depth training.
U.S. Pat. No. 5,148,961 to Humbert et al. discloses a hybrid soldering technique which includes a narrow solder wave from a solder pot and selectively applies this wave to a circuit board through a pallet containing a solder mask for the narrow portion of the board which is exposed to the solder pot wave. The pallet mask includes through-holes which correspond to the areas of the board on which solder is to be deposited, and recesses in the mask to house and protect components which would otherwise be exposed to the solder. The pallet is also provided with an inverted "V" shaped channel on the pallet's underside to prevent solder from splashing onto other parts of the circuit board. Unfortunately, this solution still requires a separate soldering device apart from a wave soldering machine. Also, this solution is not suited for or capable of simultaneously and selectively soldering two or more areas of a circuit board which are not located in the same narrow area of the board, since the entire board cannot be simultaneously passed through the wave.
Another solution is posed in U.S. Pat. No. 5,617,990 to Curtis C. Thompson, Sr. The patent to Thompson discloses a shield for use in wave soldering processes to selectively affix solder to an area of circuit board, having one or more electronic components on the solder side of the board. The shield includes a generally planar base member of a low thermal conductivity. One or more solder flow openings are formed through the base member extending from upper side to lower side. Solder flow openings are positioned to align with the selected areas of circuit board to which solder is to be affixed. One or more recesses are provided in the upper surface of the base member to receive and shield the electronic components on the solder side of the board.
The patent to Thompson also discloses a base plate or pallet made from a heat and solder resistant material sold under the tradename Delmat. The material is a composite material which is machinable and is capable of maintaining its mechanical properties at soldering temperatures. The material has a low thermal conductivity and may be specified in an electrostatic dissipative version which is suitable for electrostatic discharge sensitive applications.
Pallets formed of this material, while extremely durable, is relatively expensive due both to the cost associated with the material and the costs associated with machining the material to form the pallet. Therefore, pallets formed of this material tend to be suitable for production runs involving a relatively high number of units. In those instances involving relatively low numbers of assembled units, pallets formed in this manner and using this material may not be economically feasible.
What is needed is selective solder pallet formed of a relatively inexpensive material which allows a work piece, such as a circuit board, to be soldered using a standard or conventional wave soldering machine.
Objects of the invention therefore include the provision of a reusable pallet for protecting selected areas of a work piece from molten solder and heat as the shield and work piece are passed through a solder wave, the pallet being formed of a relatively inexpensive material. An additional object of this invention is to provide a reusable pallet formed of a material which permits the use of relatively low-cost forming methods. Still another object of the invention is to provide a solution which can be used with existing wave soldering devices to selectively apply solder to a work piece.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.